The subject matter disclosed herein relates generally to synchronous machines and, more specifically, to systems and methods for use in monitoring the physical condition of a synchronous machine.
At least some known synchronous machines are used with combustion machines, such as combustion engines, including gas turbine engines. At least some known synchronous machines are rotatably coupled to a gas turbine engine via a generator drive shaft such that the synchronous machine is operated as a generator during electric power generation operation of the gas turbine engine. Many known synchronous machines are also operated as a starting motor to accelerate the generator drive shaft and the associated gas turbine engine to a predetermined speed during start-up operations.
Known synchronous machines include a stationary stator and a rotatable rotor that are oriented such that the stator circumscribes the rotor. At least some know rotors include a plurality of windings that extend axially along the length of the rotor. Each winding includes two inter-turn portions that each wrap around the two far ends of the rotor. The windings are separated from each other to reduce the probability of a short circuit of two adjacent inter-turns that may develop over a period of time as rotational forces are exerted on worn windings. However, despite such orientation, some known rotors may still develop an inter-turn short over time.
Inter-turn shorts create a decreased number of ampere-turns that result in an unequal flux distribution within the synchronous machine. Such a decrease in ampere-turns requires the field current transmitted through the rotor windings be increased to maintain a predetermined torque and/or load on the synchronous machine. The increased current in the windings increases the temperatures of the rotor windings, and the temperatures may approach predetermined operating parameters. Also, the portion of the rotor pole that includes the shorts may not receive as much field current as the portion of the rotor that does not have any shorts. Such an imbalance in field current may result in the portion with the shorts having a lower temperature than the portion of the rotor pole without the shorts. Moreover, the portion of the rotor with the higher temperature tends to elongate axially more than the portion with the lower temperatures and may induce axial bowing forces into the pole. The induced bowing forces may bend the rotor sufficiently to induce torque pulsations that result in rotor pole vibration. Moreover, rotor bowing may cause rotor bearing temperatures to increase.
Typically, inter-turn shorts on the rotor pole are a slow-developing incipient fault and may not be detected until the fault becomes severe. Detecting and/or predicting an inter-turn short or any other slow-developing fault in a synchronous machine is very difficult without monitoring the flux density inside the synchronous machine when it is operating. The use of external measurement means, including flux probes, requires additional and expensive measurement hardware and software. Other external measurement means, such as boroscopic inspections, can only be performed when the machine is removed from service, thereby increasing potential losses of electric power generation revenue and increasing costs of shutdown maintenance operations.